What time are you people even doing Castrum for your datacenter? I'm on primal, and with the exception of like now 4-7 AM EST weekdays, the instances I get later are usually full, or fairly filled. Many of the times I've ran cluster farm groups, people have to keep trying to get into the instance before I can invite/re-invite them. I was just killing time leveling my MNK in there with FATE farm. Did a CE with like 16 people, half of them dead, completed it fine. Certain CE's can be difficult, but they can still be completed if people know what they are doing. Mettle means nothing, I respawned 3 times because I didn't feel like waiting for a revive from the party I was in, and I could just get my gold for my previous contribution and move on to another FATE. I just left Borzja with my mettle capped at level 15. Mettle loss is meaningless.

You definitely shouldn't be going into Castrum and coming out with a net loss of mettle, but losing it is overall not a big deal at all. The big deal is that time investment vs. reward. Always a waste of time in there when yeah, you can just earn more outside of the instance for the same time investment.